I'll have to take your word for it... I won't be able to get on for at least a few more hours so its purely conjecture on my part. I just saw that there is scheduled maintenance going on (on US servers at least), and with the implication of the blue post being that it would occur at dawn I suppose I presumed incorrectly.
Buff has been active since 11PM GMT+1 (London Time) yesterday in EU server. Doesn't seem to have any kind of XP advantage, there are however a lot of chests and a lot of resplendent chests so you could get lucky in terms of items!
The cow you need to kill seems to appear in act 1, I have so far seen it in the cave where the spider queen is and in Leorics passage. Not sure if its only act 1 that it spawns though.
It's pretty nice, lots of resplendent chests to open, the clickable dead bodies seem to drop a fair amount of gold as well. The Cow Queen is a bit disappointing though, after all the loot spam from chests she seems to only be dropping a single item.